diff --git a/aiohttp/http_writer.py b/aiohttp/http_writer.py
index c3216137b5d..6d02480c637 100644
--- a/aiohttp/http_writer.py
+++ b/aiohttp/http_writer.py
@@ -148,8 +148,8 @@ def set_transport(self, transport):
         self._transport = transport
 
         if self._buffer is not None:
-            for b in self._buffer:
-                transport.write(b)
+            for chunk in self._buffer:
+                transport.write(chunk)
             self._buffer = None
 
         if self._drain_waiter is not None:
@@ -160,10 +160,9 @@ def set_transport(self, transport):
     async def get_transport(self):
         if self._transport is None:
             if self._drain_waiter is None:
-                self._drain_waiter = create_future(self.loop)
+                self._drain_waiter = self.loop.create_future()
             await self._drain_waiter
 
-        assert self._transport is not None
         return self._transport
 
     @property
@@ -195,7 +194,6 @@ def _write(self, chunk):
 
         # see set_transport: exactly one of _buffer or _transport is None
         if self._transport is not None:
-            assert self._buffer is None
             self._transport.write(chunk)
         else:
             self._buffer.append(chunk)
@@ -272,7 +270,7 @@ async def write_eof(self, chunk=b''):
         self._transport = None
         self._stream.release()
 
-    async def drain(self, last=False):
+    async def drain(self):
         if self._transport is not None:
             await self._stream.drain()
         else:
diff --git a/aiohttp/web_fileresponse.py b/aiohttp/web_fileresponse.py
index 37b0f6c5fff..fdb68916ebf 100644
--- a/aiohttp/web_fileresponse.py
+++ b/aiohttp/web_fileresponse.py
@@ -19,7 +19,7 @@
 class SendfilePayloadWriter(PayloadWriter):
 
     def __init__(self, *args, **kwargs):
-        self.__buffer = []
+        self._sendfile_buffer = []
         super().__init__(*args, **kwargs)
 
     def _write(self, chunk):
@@ -27,7 +27,7 @@ def _write(self, chunk):
         # _buffer, and nothing is written to the transport directly by the
         # parent class
         self.output_size += len(chunk)
-        self.__buffer.append(chunk)
+        self._sendfile_buffer.append(chunk)
 
     def _sendfile_cb(self, fut, out_fd, in_fd,
                      offset, count, loop, registered):
@@ -62,8 +62,9 @@ async def sendfile(self, fobj, count):
         offset = fobj.tell()
 
         loop = self.loop
+        data = b''.join(self._sendfile_buffer)
         try:
-            await loop.sock_sendall(out_socket, b''.join(self._buffer))
+            await loop.sock_sendall(out_socket, data)
             fut = loop.create_future()
             self._sendfile_cb(fut, out_fd, in_fd, offset, count, loop, False)
             await fut